A recent series of studies from our laboratory concentrated on describing the relative role of age, cardiovascular endurance, and the menstrual cycle in determining women's ability to tolerate work in hot environments. Our results indicated that fluctuations in hormonal levels plays a minor role and that most of the variability between individuals was due to differences in age and fitness levels. While women of any age can and do react to acute exposlure to heat with signs of physiological strain, the proportion of susceptible individuals was higher at each end of the age continuum. Prepubertal children and postmenopausal women were more likely to be at risk than women between these age extremes. Their lower tolerance to exercise in the heat was related primarily to circulatory instability and a decreased sensitivity in the sweating mechanism. The purpose of the proposed series of studies is to investigate these mechanisms in detail. The relative contribution of central and peripheral mechanisms in effecting changes in the sensitivity of the sweating mechanism, patterns of body fluid shifts, and responses to acclimatization procedures will be quantified and evaluated for age and/or fitness related differences in response. The results of such studies will help us understand how females respond to the stress of working in hot environments as well as providing insight into some of the unanswered questions in the area of thermoregulation.